Mechanical seals between a rotatable shaft and its stationary supporting housing, may include annular sealing rings respectively sealed and connected or keyed relative to the shaft and to the housing, and having annular sealing faces disposed facing one another. The relative rotation of the shaft and housing takes place between these sealing faces; and they are adapted to cooperate closedly with one another to minimize fluid leakage between the faces, from a high pressure at one radial edge of the sealing faces to a lower or atmospheric pressure at the opposite radial edge.
At least one of the sealing rings generally is movable axially of the shaft, and biased by springs or the like, to urge the annular sealing faces closely proximate, or even substantially against one another, particularly when the shaft is not being rotated. However, to reduce wear between the sealing faces, as they are being moved relative to one another when the shaft is being rotated, it is intended that a small fluid film gap be established between the sealing faces. The gap thickness is determined when the forces acting on the moving ring member balance; such forces including the hydrostatic and hydrodynamic fluid pressure acting on the opposite faces of the member, and the force of the closing spring. The fluid forces may be from the contained fluid, although a secondary lubricating fluid could also be used.
When properly designed, the fluid film gap between the sealing faces may be meaured in tenths-of-thousandths of an inch (0.0001"), and the leakage through the gap is minimal. This sealing gap nonetheless eliminates or minimizes direct contact between the sealing faces, to significantly reduce wear of such faces; and further to significantly reduce frictional drag, heat buildup and power consumption of the mechanical seal. With an adjacent sealing faces be made to very close tolerances; and which sealing faces, during operation, remain true and parallel relative to one another.
Precise positioning of the seal rings with respect to each other is particularly critical in dry running gas seals. Only the contained fluid itself, typically a gas, is used. Gas generally is not an effective lubricant and control of hydrostatic and hydrodynamic forces is relied on to provide a fluid film gap between the sealing faces, to avoid direct surface contact. Even so, the gap between the sealing faces must be very small to contain a gas, without allowing excessive leakage.
A successful design of a dry running seal has a plurality of circumferentially spaced shallow radial grooves provided in one of the sealing faces. These grooves extend spirally from the high pressure edge part-way across the face toward the low pressure edge. The pressure of the contained fluid at the high pressure edge, is present via the grooves to an interior region of the sealing faces, to provide for a hydrostatic pressure tending toward separation of the sealing faces. As the faces rotate, the pumping action of the grooves creates a hydrodynamic force separating the faces to some gap, until counterbalanced by the forces of the springs and the opposing fluid forces tending to close the gap. This avoids direct contact between the sealing faces. U.S. Pat. Nos. 3,499,653 and 4,212,475 disclose specific embodiments of mechanical dry running gas seals, and the teachings of such patents are incorporated herein by reference.
In a mechanical seal of the type disclosed in the above mentioned patents, the sealing and/or film leakage gap is dynamic and changing. When the shaft is not rotating the seal faces are in contact. As the shaft rotates the pumping action of the seal face grooves causes the faces to move apart. Other factors such as axial shaft movement, thermal distortion of parts, pressure variations, etc. require that one seal face accommodate axial movement. One sealing ring is, therefore, moveable axially along the shaft. As noted the gap for a dry running seal may be measured in tenths-of-thousandths of an inch (0.0001"), and the movement of the one sealing ring is comparable. Any resistance hindering the moveable sealing ring in this very small range detracts from the sensitivity of the mechanical seal. Excessive resistance preventing compensating movement of the movable ring may hold the gap between the sealing surfaces too small, causing wear, overheating and/or even destruction of the adjacent sealing faces; or may keep an open gap from closing, allowing excessive leakage.
Of imporatnce also is the fact that forces on the sealing faces must be reasonably uniform and symmetrical of the sealing faces and the rotational axis of the sealing faces. This means that the sealing faces must be properly centered relative to one another. If the sealing faces are offset, from a coaxial position, or caused to move out of parallel relative to one another, the sealing faces at one local region may be forced too close together or even contact one another, while they may be spaced apart excessively at another spaced local region . . . to have both wear and leakage problems. There factors are augmented by any non-coaxial position of the seal rings.
Centering of the relatively rotating sealing rings is important to seal performance and seal component life. If one ring is disposed in a position offset from the axial centerline of the other, unequal loading can occur, for example, from the fluid pumped between the sealing faces by the spiral pumping grooves.
In prior designs with seals exposed to the high pressure to be sealed at the outside diameter of the seal rings, it has been adequate to center the seal rings at the inside diameter. For operating pressures and temperatures previously experienced, such an approach had been adequate. However, as pressure and temperatures experienced in seal environments have become more severe, centering of the seal rings becomes more significant and critical.